1. The Field of the Invention
This invention relates to a method for preparing a diene copolymer. More particularly, this invention relates to a method for preparing a copolymer useful for tire production by mixing isoprene and 1,3-butadiene at a predetermined weight ratio to form a living polymer, coupling the living polymer with a multi-reactive polysiloxane, and subsequently modifying the uncoupled active ends of the living polymer with an organic amine compound.
2. Related Prior Art
Compared with the diene polymer prepared with a Ziegler-Natta catalyst, the diene polymer synthesized using an organolithium initiator is readily more controllable in vinyl content and more excellent in wet traction and rolling resistance. Especially, the polymerization of a diene monomer using an organolithium initiator introduces multiple functional groups to the ends of the polymer to enhance compatibility with silica used as a reinforcing material for tires, reducing rolling resistance or increasing wet traction. The term “rolling resistance” as used herein has a close connection with the rate of fuel consumption of a running vehicle. With an increase in the rolling resistance, the friction force of vehicle tires from the road surface increases to deteriorate the rate of fuel consumption of the vehicle. Otherwise, with a lower rolling resistance, the rate of fuel consumption of the vehicle becomes higher. The rolling resistance is generally expressed in terms of a tan δ value at around 60° C. The lower tan δ value represents a tire material having more excellent in rolling resistance.
Another significant dynamic property of a tire material is wet traction, which is closely connected with the braking performance of a running vehicle body. With an increase in the wet traction, the friction force of vehicle tires from the road surface increases to acquire a higher braking performance. Contrarily, the braking performance deteriorates with a decreased wet traction. The wet traction is generally expressed in terms of a tan δ value at around 0° C. The higher tan δ value represents a tire material having more excellent in wet traction.
In the tire materials, the rolling resistance is the opposite property to the wet traction. Therefore, the tire material that satisfies both the opposite properties is considered to be excellent as a material for tire production.
As an approach to improve the rolling resistance of a diene polymer by decreasing a heat build-up value (based on the fact that, with a decreased heat build-up value of the polymer, the rolling resistance becomes lower to increase the rate of fuel consumption of the tire material), there has been suggested an anionic polymerization method for preparing a diene polymer containing a tertiary amino group (Japanese Patent Laid-Open No. 1989-101344), an alkylsilyl group (Japanese Patent Laid-Open No. 1989-188501), or a halogenated silyl group (Japanese Patent Laid-Open No. 1993-230286). However, the method does not lower the heat build-up value so much as expected and disadvantageously deteriorates the processability of the polymer when admixing it with silica.
Other examples of the method for improving the rolling resistance by reducing the heat build-up value of the diene polymer are the polymerization methods using a silane compound as a coupling agent as disclosed in Japanese Patent Laid-Open Nos. 1991-252431 and 1991-252433. However, these methods cannot also improve the heat build-up value of the diene polymer so much as expected, requiring the use of an excess of a very expensive silane compound to improve the heat build-up value to an expected level.
On the other hand, there are many approaches to balance the opposite properties of the tire material, rolling resistance and wet traction. For example, U.S. Pat. Nos. 4,834,120 and 5,137,998 describe that a polymer having multiple glass transition temperatures as synthesized using an anionic initiator can be improved in both rolling resistance and wet traction. More specifically, the method disclosed in the cited inventions includes synthesizing a polymer having a glass transition temperature in a defined range at a first reactor and then controlling the polymerization condition to make the polymer have a second glass transition temperature. However, a commercial production of the polymer having at least two glass transition temperatures by this method demands high complexity of the synthesis process, elongation of reaction time, and increases in the numbers of polymerization facilities, resulting in a deterioration of productivity.
For improving compatibility of rubber with carbon black, there has been proposed a method of modifying the ends of the polymer molecule with an organic amine compound such as amino benzophenone to provide a rubber composition superior in dynamic and mechanical properties to the existing rubbers (U.S. Pat. No. 4,555,548). However, the rubber synthesized by this method is known to have poor processability during admixing. The use of such a rubber as a tire material hardly provides compatibility with silica, deteriorating mechanical and dynamic properties of tire products. For that reason, the use of the rubber in tire production may cause many difficulties (U.S. Pat. Nos. 4,555,548 and 5,219,945).
In addition, there is an approach to maximize the affinity of rubber to a reinforcing material by polymerizing the rubber in the presence of a functional initiator and then substituting the ends of the rubber molecule with an amine compound or a silicon compound. However, the rubber thus prepared has poor storage stability because of its high cold flow at the ambient temperature (U.S. Pat. No. 6,133,388).
To solve the aforementioned problems, there are many attempts to treat the ends of the anionic living polymer with ethylene oxide (J. Polym. Sci., Part A: Poly. Chem., 26, 2031 (1988)), diphenylethylene (J. Polym. Sci., Part A: Polym. Chem., 30, 2349 (1992)), or N-(benzylidene)trimethylsilylamine (Makromol. Chem., 184, 1355 (1983)). However, the methods still have a limitation in acquiring sufficient compatibility with inorganic filler.
In the preparation of a rubber material for tire production using an anionic polymerization initiator, the coupling agent increases the molecular weight of the polymer and broadens the molecular weight distribution, thus enhancing the mechanical properties of the polymer and improving the processability. The use of a proper coupling agent improves the affinity and compatibility of the rubber material with filler, resulting in enhanced dynamic properties (i.e., increasing wet traction and reducing rolling resistance) required for tires. To achieve this purpose, there have been developed numberless different coupling agents. For example, UK Patent No. 1103939 discloses a method for preparing a polymer using CO2 or CS2 as a coupling agent. However, the coupling agent exhibits a low efficiency and has some problems that the concentration of CO2 is not readily controllable and that CS2 forms a sulfide compound as a byproduct contaminating the polymer product. In like manner, U.S. Pat. Nos. 4,039,633 and 3,468,972 disclose a method for preparing a polymer using a 1,3,5-benzenetricarboxylic acid trihalogen compound and a polyepoxide compound as a coupling agent, respectively. In both cases, the coupling number of the polymer is not readily controllable, and byproducts are formed to discolor the polymer or give out an odor.
Other examples are those methods for preparing a polymer using silicon halide, siloxane, silyl amine, or silyl sulfide as a coupling agent, as disclosed in U.S. Pat. Nos. 3,244,664, 3,692,874, and 3880954, respectively. These methods also form byproducts such as alcohol, amine, thiol, or the like, which act as a poison of the initiator that makes the production yield and the molecular weight of the polymer uncontrollable.
In case of using a tin coupling agent for the ends of a copolymer prepared in the presence of an organolithium initiator by solution polymerization, the compound rubber with carbon black can be improved in both rolling resistance and wet traction due to high compatibility of the tin compound with carbon black. However, the use of the tin compound as a coupling agent provides a weak bond between the tin compound and the polymer (i.e., Sn—C bond), which is readily broken by the physical force and additives during admixing, resulting in a deterioration of mechanical properties. For that reason, the tin-coupled rubber is restricted in its use when admixing with silica where the processing conditions are more rigorous than those used with carbon black (U.S. Pat. No. 4,397,994).